1. Field of the Invention
The present invention relates generally to direct sequence spread spectrum communication systems, and more specifically to a multi-stage serial interference cancellation direct sequence CDMA (code division multiple access) receiver.
2. Description of the Related Art
As described in "Serial Canceler Using Recursive Channel Estimation By Pilot Symbols for DS-CDMA", M. Sawahashi et al, Institute of Electronics, Information and Communication Engineers (Japan), Technical Report, SAT 95-14, RCS95-50 (1995-07), a multi-stage serial canceler uses recursive channel estimation to estimate replicas of interference from other user channels using despreading codes. The estimated replicas are iteratively summed and combined with time-coincident channel signals in each cancellation stage. The power levels of user channels are detected and their ranking determined. The ranking information is then stored in a store-and-shift circuit where it is shifted along as cancellation proceeds from one stage to the next. The ranking information is supplied to the interference cancelers of each stage to allow them to select appropriate despreading codes. Since the store-and-shift circuit is required to hold users' timing and power level information, a significant amount of memory and complex control circuitry are required for precisely feeding the stored information to all cancellation stages. If the number of user channels and/or the number of cancellation stages increase, the central control circuitry would become considerably complex.